Bump PHPStan rules to level 9 (#2551)

After using PHPStan for a while, our type coverage is now far more
complete and most of the worst issues have been fixed. The codebase is
now in a state where many of the errors reported by PHPStan level 9 are
useful, instead of being a nuisance. If this proves to be incorrect
after a few weeks or months of testing, we can reduce our rule level
back down to level 8.